Herpesvirus infections in humans can be caused by different human herpesviruses, the most common being herpes simplex virus and varicella-zoster virus. There are also many animal herpesviruses.
Following a primary infection with herpes simplex virus or varicella-zoster virus, the virus establishes latency in the sensory nerve cells for the rest of the patient's life and can subsequently be reactivated repeatedly. Following a reactivation in the nerve cell, the virus is transported through the nerves to the skin and subsequently a lesion develops. One characteristic of herpesvirus infection is the inflammation which follows immediately upon an outbreak of virus replication. The inflammation contributes to all symptoms associated with herpesvirus recurrence including redness, swelling, itching and pain as well as lesions.
Herpes simplex viruses can be divided into two serotypes, HSV type 1 (HSV-1) and type 2 (HSV-2), the clinical manifestations of which range from benign self-limiting orofacial and genital infections to potentially life threatening conditions such as encephalitis and generalized neonatal infections.
Oral-facial HSV infections are primarily caused by HSV-1. Following a primary infection in childhood the virus becomes latent. After reactivation a recurrent oral-facial HSV infection develops, which is more commonly known as a cold sore. About half of the patients experience prodromal symptoms such as pain, burning or itching at the site of the subsequent lesions. The condition is generally rapidly self-limiting and a typical episode will heal in around 10 days from the first symptoms. Viral replication in the lip is initiated early and maximum virus load is attained 24 hours after the onset of the reactivation. The virus concentration is then dramatically reduced and typically virus cannot be isolated 70-80 hours after the onset.
The clinical presentation of genital HSV infections is similar to the oral-facial infections with some important exceptions. Genital HSV infections are most often caused by HSV-2 and following the primary infection the virus will latently infect sensory or autonomic ganglions. Reactivation will produce the local recurrent lesions on or near the genitals that are characteristic of the herpes infection.
A primary infection with varicella-zoster virus (VZV) causes chicken-pox. Like HSV, VZV becomes latent following the primary infection and can be activated as herpes zoster later on in life. Zoster usually results in skin rash and intensive acute pain. In 30% of the patients, the pain can be prolonged and continue for weeks or months after the rash has cleared up.
HSV and VZV may, in addition to mucous or cutaneous manifestations, also cause keratitis in the eyes. This condition is also recurrent and may cause blindness.
There are a number of antiviral agents which are active against the human herpesviruses. There has, however, so far been limited clinical success in the treatment of recurrent herpesvirus infections.
Foscarnet, that is the hexahydrate of the trisodium salt of phosphonoformic acid or sodium phosphonoformate hexahydrate, and acyclovir are well-known antiviral compounds, which as topical formulations have been extensively tested against recurrent herpes simplex virus infections in clinical trials with only a moderate degree of success. Acyclovir has, in addition, been tested as an oral formulation against recurrent HSV infections with significant but limited clinical effect if used after the appearance of symptoms. As a result of such a treatment the healing time will be shortened by no more than approximately one day. The inhibitory effect of foscarnet, as well as that of acyclovir, against herpes viruses in vitro is, however, high.
Standard-dose acyclovir shows little effect against VZV infections in the clinic. High-dose acyclovir shows significant but limited effect on zoster lesions if treatment is initiated within 72 hours from the appearance of the first symptoms.
Other antiviral substances, which have been used topically and which exhibit an inhibitory effect against HSV in vitro, are for example adenine arabinoside (ara-A, vidarabine), arabinosyladenine-monophosphate (ara-AMP), lobucavir (bishydroxymethylcyclobutylguanine, BHCG), brivudine (bromovinyldeoxyuridine, BVDU), desciclovir, famciclovir, cidofovir (HPMPC, GS504), idoxuridine, netivudine (zonavir, BW882C87), penciclovir, PAA (phosphonoacetate), PFA (phosphonoformate), sorivudine (brovavir, BV-araU), trifluridin (trifluorothymidine, TFT), tromantadine, valacyclovir, virend, 1-docosanol (lidakol), 348U87, 2242 (2-amino-7-(1,3-dihydroxy-2-propoxymethyl)purine), HOE 961, civamide (capsaicin), PMEA (9-(2-phosphonylmethoxyethyl)adenine), peptide T, BILD 1263, CRT.
Clinical primary infections with human herpes simplex viruses differ in a number of important aspects from subsequently reactivated infections. The viral shedding period is longer in the primary infection (about 10 days in labial and 3 weeks in genital infection) compared with reactivated infection (3-4 days for both labial and genital infections). Following termination of the viral shedding period in primary infections the lesion will heal in a few days while in the case of reactivated infections, the inflammation continues after viral replication has ceased and the clinical symptoms will remain for another week.
Obviously a reduction of virus multiplication in itself will not substantially alter the clinical course of a recurrent herpes infection. It is, therefore, not surprising that antiviral drugs when tested in clinical trials show a more substantial effect against a primary infection as compared with reactivated infections, such as recurrent herpes labialis or genitalis. Because of the rapid self-limiting nature of the virus shedding period in recurrent HSV infection the improvement of only one day healing time obtained in clinical trials with antiviral drugs is not surprising.
Different antiinflammatory agents have been tested to treat the inflammation that accompanies the recurrent infection, but only with limited success. Traditionally, inflammatory conditions in the eye, such as keratitis, have been treated with steroids. Even though this type of compounds is known to potentially promote herpesvirus replication steroids have been used in severe cases, for instance to save the patients vision. This practice has been controversial.
In summary, there has been little clinical success in the treatment of recurrent herpesvirus infections even with the most potent antiviral drugs. There is, thus, a great need for effective drugs and methods of treatment for recurrent herpes infections.